In a variety of fields it is useful to sample signals at a high rate in order to determine the time variations in intensity, phase or other features that occur in a waveform over time. One example of this sampling is in the use of fluorometers, where precise fluorescence intensity data may be gathered as part of a variety of detection tasks. In particular, it is desirable to record the time rates of decay of the fluorescence produced by one or more fluorophores as a result of illumination by a short pulse of light. Another example is in observing transmission phenomena, where changes between a transient input signal applied to a system and the corresponding transient output signal are of interest. A further example is radar and similar systems in which a signal is emitted and it is of interest to capture the signal resulting from a reflection. If the sampled signals are digitized, it greatly facilitates analysis of the resulting data.
There exist systems that perform continuous digitizing of sampled analog data. If the sampling rate must be high, then because all the components involved in sampling and analog to digital (A to D) conversion must operate at the same high rate to avoid massive analog memory requirements, the systems are very expensive. In some circumstances the high sampling rate needs to occur only during a short sampling window corresponding to a transient event. This provides an opportunity to sample at a high rate and perform digitizing at a slower rate. This limits memory and may permit cheaper components to be used for slower functions. Such systems are known as Fast-in Slow-out (FISO) systems, e.g., U.S. Pat. Nos. 4,833,445 and 6,091,019.
With most FISO systems challenges remain. When sampling data from an event occurs at near gigahertz (or higher) rates, large amounts of samples must be stored and processed. Often the interval for processing the samples is limited by the occurrence of the next event and its corresponding sampling activity. Thus, there are trade-offs between and among the sampling rate, the duration of the sampling window, the speed of the A-to-D conversion, the frequency of occurrence of the sampling window, the amount of data that can be collected and the amount of digitized data that can be passed on for downstream processing. If the duration of the sampling window is short, then the window on the event observed is narrow. Thus, when a high sampling rate is required, a shortening of the sampling window can help limit the downstream data processing but may also mean that less than adequate observations are made. Designs achieving increases in speed or amount of data collected almost always involve cost increases or power demands that limit applications for the design.
It is unusual to capture long waveforms using a high sampling rate. Such capture typically requires relatively expensive components. In the fluorescence situation, the most expensive elements are the light source (usually a laser) and the digitizer, usually a digital storage oscilloscope. The relatively high cost has limited the use of such equipment.
In applications that measure induced fluorescence, a certain period of time is needed to acquire a statistically significant number of emitted photons. One of the factors determining the measurement time is the average intensity of the excitation source. In the case of a pulsing microchip laser, which delivers high-intensity pulses, acquiring the response to a single sub-nanosecond pulse may be all that is necessary. In some applications, it is desirable to use low-cost LED-based excitation sources as an alternative to the microchip laser. In the applications where there is a modulated LED, which may emit relatively long, low-intensity pulses, a longer period of time is needed to acquire sufficient emitted photons for adequately defining a waveform. However, in an application such as a bio-aerosol warning system, the sample under investigation is illuminated for a limited period of time, typically on the order of 1 millisecond. Consequently, for LED excitation, the combined constraints of intensity and time can result in a very limited number of emitted photons. Therefore high efficiencies are desired for coupling, collection and detection.
Unless there is a large amount of storage, the most recently captured waveform must be read out before another waveform can be captured. The conversion and readout time can be much longer than the capture time. This is not a problem for short, widely-spaced events, but LED excitation can be essentially continuous. In this case, much of the available information may be lost, because the sampling and processing cannot keep up with the waveforms produced by near-continuous excitation.
It would be desirable to develop a system and method for capturing analog samples of data signals that could provide a high rate of sampling and efficient delivery of digital data derived from the captured samples. Other desirable features are a high degree of accuracy and lower cost than conventional devices. A further desirable feature is the ability to efficiently acquire a statistically significant number of emitted photons when low intensity excitation sources are used or other factors reduce the number of detected photons.